The system of the IEEE 802.16e that is undergoing international standardization with respect to broadband wireless access systems, does not have a layered architecture comprised of a HLR (Home Location Register), a VLR (Visitor Location Register), a MSC (Mobile Switching Center), a BSC (Base Station Controller), a RNC (Radio Network Controller), and the like, as in the existing 2 G and 3 G mobile communication systems, but instead, is comprised of only a MSS (Mobile Subscriber Station), a BS (Base Station) and a ASA (Authentication Service Authorization).
The broadband wireless access system defines the protocols of the MAC (Medium Access Control) layer and the PHY (PHYsical) layer that are common between the base station and the mobile station (MSS). FIG. 1 depicts an exemplary protocol layer architecture for a broadband wireless access system.
The related art procedures to perform handover of a mobile terminal from one base station to another base station are as follows:
1) Cell Selection
The mobile station can obtain information related to neighbor base stations by using a neighbor base station advertisement message (e.g., MOB_NBR-ADV) that is broadcast by a serving base station or by using scanning.
2) Handover Decision and Initiation
The decision to perform handover of a mobile station to another base station can be initiated by the mobile station or by the current serving base station.
3) Target BS Scanning
The mobile station should perform scanning on the target base station in order to obtain downlink and uplink channels and their synchronization.
4) Network Re-entry
The mobile station and target base station should perform ranging in order to allow network re-entry for the mobile station.
5) Termination of Service
Upon performing handover to another base station, the mobile station ends all services with the previous base station.
In the related art, the messages related to handover will be explained as follows:
1) MOB_MSHO-REQ message
To request handover, the mobile station sends a MOB_MSHO-REQ message to the base station. The MOB_MSHO-REQ message includes IDs of target base stations for handover recommended by the mobile station, preamble index values, CINR values, HO-IDs, and the like.
2) MOB_BSHO-RSP message
Upon receiving the MOB_MSHO-REQ message, the base station sends a MOB_BSHO-RSP message to the mobile station as a response.
3) MOB_BSHO-REQ message
This is a message sent to the mobile station, by the base station that is currently serving that mobile station, in order to request handover to another base station. The MOB_BSHO-REQ message can include information about neighbor base stations that have been recommended by the base station.
4) MOB_HO-IND message
After receiving the MOB_BSHO-RSP or the MOB_BSHO-REQ messages and upon determining a target base station to which handover is to be ultimately performed, the mobile station releases the connection with the currently serving base station, and this MOB_HO-IND message is sent when handover is about to be performed to another base station.
FIG. 2 depicts the procedures for performing handover in a broadband wireless access system according to the related art.
(1) The mobile station obtains related information of the neighbor base stations through scheduling procedures and neighbor base station broadcast messages.
(2) The mobile station transmits to a currently serving base station a message for requesting handover. Here, recommended base stations based upon the information obtained through the scanning procedure are included in the transmission.
(3) Upon receiving this request message, the currently serving base station transmits, to the recommended base stations, a handover pre-notification message (HO-pre-notification) that include values, such as a mobile station identifier, the required bandwidth, connectivity parameters, and the like.
(4) In response, the recommended base stations transmit a handover pre-notification response message (HO-pre-notification-rsp).
(5) Upon checking the response message of the recommended base stations, the currently serving base station determines a target base station that is appropriate for the mobile station and sends a handover confirm message (HO-confirm) and sends a handover response message (MOB-HO-RSP) to the mobile station.
(6) The mobile station finally sends a MOB_HO-indication message to the base station for performing handover.
(7) The mobile station performs ranging with the base station to which handover is to be performed.
As explained above, in the related art, the messages used by the mobile station to perform scanning that are transmitted and received with the base station are defined, but the operations actually related to scanning are not defined for the protocol stack (layers) within the mobile station and the protocol stack (layers) within the base station. Namely, there are no definitions for the primitives with respect to information transferred between the MAC layer and upper management entity via the NCMS (Network Control Management System), within the protocol stacks of the mobile station and the base station, respectively.